Abstract

A formulation of an LPV control problem with regional pole placement constraints is presented, which is suitable for the application of a Full Block S-Procedure. It is demonstrated that improved bounds can be obtained on the induced 𝐿2 norm of closed loop systems, while satisfying pole placement constraints. An application consisting in the 6 degrees of freedom (DOF) control of a space vehicle is developed as an example, with hardware in the loop (HIL) simulation. This shows that the method is appealing from the practical point of view, considering that the synthesized control law can be implemented satisfactorily in standard flight control systems. Conclusions with remarks towards the practical use of the method are presented as well.

1. Introduction

An interesting technique that has allowed linear parameter varying (LPV) control synthesis algorithms to obtain less conservative performance bounds is given the name of Full Block S-Procedure (FBSP). See [1, 2] and references therein. The formulas in [3, 4], render synthesis conditions combining the techniques of [1, 5], to develop a generalized framework for LPV control, based upon parameter dependent Lyapunov functions (PDLFs) and full block multipliers (FBMs). Yet these results provide linear matrix inequality (LMI) constrains with an infinite number of inequalities. In practice, one must resort to gridding the parameter variation set in order to apply the methods.

Subsequent work presented in [6] makes focus in particular form of PDLFs, namely, PDFLs that depend on the parameter in a linear fractional fashion, for systems whose open loop state space matrices depend on the parameter in a linear fractional way as well. In the sequel we will call the former LFT PDLFs and the latter LFT systems. From the practical point of view, the technique, which is based upon [4], is most appealing considering renders a set of constraints with a finite number of LMIs.

Another reference that is relevant from the practical point of view in LPV control is given by [7]. This work compares the results of [5] with an extension to LPV systems of the results in [8]. Helpful hints are given, in order to obtain LPV controllers that can be implemented in practice. The synthesis methods seek to establish stability and performance making use of PDLFs.

It must be recalled that synthesis conditions like the ones presented in [5] require a first step in order to establish feasibility of the problems and a second step to calculate the controllers. Under certain rank restrictions imposed on the state space matrices of the augmented plant, closed form expressions can be given for the calculation of the controllers. As opposed to this case, in [8] like results, an optimization is directly carried out, on a set of variables which are equivalent to the final state space matrices of the controller, through a nonlinear change of variables, which is the approach followed in this work.

It is rather frequent, in the application of LPV methods, that feasible problems with acceptable 𝛾-performance indexes (see [5, 9] for a definition) show undesirable transient response. With a slight abuse of terminology, it can be posed as LPV systems having closed loop “poles” in undesirable locations. It is actually the poles of each linear time invariant system (LTI) resulting from holding the parameter vector constant, that turn out to have a nonconvenient loci. It has been reported previously that LPV synthesis, in particular when approached through a single quadratic Lyapunov function (SQLF), shows a problem called “fast poles” (see [10]).

The aspect of transient response of systems has not been dealt with in the FBSP framework of [6]. The work presented here extends the application of the FBSP, together with LFT PDLFs, to the same kind of LFT systems [6] deals with. The proposed approach includes the possibility of having the closed loop poles of each LTI system resulting from LPV dynamics with constant parameter trajectories, to have their loci in a prescribed region. This idea is most appealing from the practical viewpoint, in order to carry out the simulation and implementation of controllers. An application to the problem of 6 degrees of freedom (DOF) control of a spacecraft is presented with numerical hardware in the loop (HIL) simulations, as an application example.

The paper is organized as follows. Section 2 presents the developed formulas for LPV control with closed loop regional pole placement and FBMs. In Section 3, the synthesis method is employed to design the 6 DOF control for a rocket. Concluding remarks are given in Section 4.

2. Synthesis Method

In this section the synthesis method of LPV control with FBMs, PDLFs, and regional pole placement constraints is presented.

2.1. Background

The set 𝒫𝑠 is such that for each 𝜃=(𝜃1,,𝜃𝑠)𝒫, |𝜃𝑖|1. On the other hand, for some 𝜈=(𝜈1,,𝜈𝑠)𝑠 with 𝜈𝑖>0, all 𝜈=(𝜈1,,𝜈𝑠)𝒱𝑠 are such that |𝜈𝑖|𝜈𝑖. An 𝑟𝛼=(𝑟𝛼1,,𝑟𝛼𝑠)𝑠 defines the sets Θ𝛼={Θ𝛼=diag{𝜃1𝐼𝑟𝛼1,,𝜃𝑠𝐼𝑟𝛼𝑠}𝜃𝒫} and ̇Θ𝛼={̇Θ𝛼=diag{̇𝜃1𝐼𝑟𝛼1,,̇𝜃𝑠𝐼𝑟𝛼𝑠}̇𝜃𝒱}. The number 𝑛𝛼=𝑠𝑖=1𝑟𝛼𝑖 is used later. In the sequel, any 𝑟𝛽=(𝑟𝛽1,,𝑟𝛽𝑠)𝑠 will be regarded as defining a number 𝑛𝛽 and a couple of sets Θ𝛽, and ̇Θ𝛽, in the same fashion as 𝑛𝛼, Θ𝛼, and ̇Θ𝛼 before. With the subindex omitted, it will be just 𝑟=(𝑟1,,𝑟𝑠)𝑠, defining 𝑛𝑝=𝑠𝑖=1𝑟𝑖, Θ and ̇Θ. Throughout this paper, systems state space matrices depend on 𝑠-dimensional parameter trajectories evolving in the set 𝜈𝒫={𝜃𝒞1(+,𝑠)𝜃(𝑡)𝒫,̇𝜃(𝑡)𝒱,forall𝑡+}.

The following lemma is crucial in order to use FBMs for LPV control.

Lemma 1 (Full Block S-Procedure). Let 𝒢(Θ)=Θ𝐺11𝐺12𝐺21𝐺22=𝐺22+𝐺21Θ𝐼𝐺11Θ1𝐺12(1) be a linear fractional transformation (LFT) where 𝐺11, 𝐺12, 𝐺21 and 𝐺22 are real matrices of compatible dimensions. Given a real symmetric matrix 𝑀, the quadratic matrix inequality 𝒢𝑇(Θ)𝑀𝒢(Θ)<0(2) holds for all ΘΘ, if and only if there exists a real symmetric full-block multiplier Π such that for any ΘΘ, 𝑇diag{Π,𝑀}𝐺11𝐺12𝐼0𝐺21𝐺22<0,𝐼Θ𝑇Π𝐼Θ0.(3)

Proof. See [6].

Remark 1. Condition (3) consists of an infinite number of constraints. Considering Θ a compact set defined by its 2𝑠 vertices, additional constraints can be added in order to turn (3) into a condition with a finite number of constraints. Namely, partition the multiplier Π as Π=Π11Π12Π𝑇12Π22,(4) and request Π22<0. Then condition (3) will be convex with respect to Θ. As a consequence, if for all vertices Θ𝑖 of Θ, the following LMI constraints 𝐼Θ𝑖𝑇Π11Π12Π𝑇12Π22𝐼Θ𝑖0(5) are satisfied, then inequality (3) will be so itself. This remark is important from the computational point of view. Notice that, while acceptable in practice, the approach can be conservative. Moreover, as observed in [6], fulfillment of constraint (3) is achieved if it is further enforced that Π11=Π22>0, with Π11, Π22 being symmetric, Π12 being skew-symmetric, and all of them being commutable with all Θ in Θ.

In order to state the control problem, consider an LFT parameter-dependent plant:̇𝑥(𝑡)𝑒(𝑡)𝑦(𝑡)=𝒜(Θ(𝑡))1(Θ(𝑡))2(Θ(𝑡))𝒞1(Θ(𝑡))𝒟11(Θ(𝑡))𝒟12(Θ(𝑡))𝒞2(Θ(𝑡))𝒟21(Θ(𝑡))𝒟22(Θ(𝑡))𝑥(𝑡)𝑑(𝑡)𝑢(𝑡),(6) where Θ(𝑡)Θ, ̇𝑥, 𝑥𝑛, 𝑑𝑛𝑑 is the disturbance, 𝑒𝑛𝑒 is the controlled output, 𝑢𝑛𝑢 is the control input and 𝑦𝑛𝑦 is the measurement for control. The underbraced state space matrices of (6) depend on the parameter 𝜃 in a linear fractional way as follows:𝐴𝐵1𝐵2𝐶1𝐷11𝐷12𝐶2𝐷21𝐷22+𝐵0𝐷10𝐷20Θ(𝑡)𝐼𝐷00Θ(𝑡)1𝐶0𝐷01𝐷02.(7) It is assumed that the LFT representation is well-posed; that is, (𝐼𝐷00Θ(𝑡)) is invertible for any allowable parameter values. It is also assumed that the triple (𝒜,2,𝒞2) is parameter-dependent stabilizable and detectable for all 𝜃𝜈𝒫. This guarantees the existence of a stabilizing output feedback LPV controller. The class of LPV controllers we are interested in is of the forṁ𝑥𝑘(𝑡)𝑢(𝑡)=𝒜𝑘Θ(𝑡),̇Θ(𝑡)𝑘Θ(𝑡),̇Θ(𝑡)𝒞𝑘Θ(𝑡),̇Θ(𝑡)𝒟𝑘Θ(𝑡),̇Θ(𝑡)𝑥𝑘(𝑡)𝑦(𝑡),(8) where 𝑥𝑘𝑛𝑘. The dimension of controller state 𝑛𝑘 is yet to be determined.

The synthesis method used in this paper is based upon the results in [6] for LPV systems and [8] for LTI synthesis with pole clustering. The following definition is taken from [8].

Definition 1 (LMI-region). A subset 𝒟 of the complex plane is called an LMI region if there exist a symmetric matrix 𝛼=[𝛼𝑘𝑙]𝑚×𝑚 and a matrix 𝛽=[𝛽𝑘𝑙]𝑚×𝑚 such that 𝒟={𝑧𝑓𝒟(𝑧)<0} with 𝑓𝒟(𝑧)=𝛼+𝑧𝛽+𝑧𝛽𝑇=𝛼𝑘𝑙+𝛽𝑘𝑙𝑧+𝛽𝑙𝑘𝑧1𝑘,𝑙𝑚.(9)

These regions make up a dense subset in the set of regions of the complex plane, symmetric with respect to the real axis. This makes them appealing for specifying pole placement design objectives.

Theorem 1 (LPV basic characterization with pole placement constraints). Let 𝑟𝒮=(𝑟𝒮1,,𝑟𝒮𝑠) and 𝑟=(𝑟1,,𝑟𝑠)𝑠 define 𝑛𝒮, 𝑛, Θ𝒮, ̇Θ𝒮 and Θ, ̇Θ as in the beginning of this section. Let 𝒮(Θ𝒮)=𝒯𝒮(Θ𝒮)𝑇𝑄𝒯𝒮(Θa𝒮) and (Θ)=𝒯(Θ)𝑇𝑃𝒯(Θ), with 𝒯𝒮Θ𝒮=Θ𝒮𝑇𝒮11𝑇𝒮12𝑇𝒮21𝑇𝒮22,𝒯Θ=Θ𝑇11𝑇12𝑇21𝑇22,(10) be two symmetric and positive definite matrix functions, where constant matrices 𝑄 and 𝑃 are yet to be found, with Θ𝒮Θ𝒮 and ΘΘ. Consider the LPV plant governed by (6), with parameter trajectories in 𝜈𝒫. Suppose that there exist parameter dependent symmetric matrices 𝒮 and such as (10), a positive real number 𝛾, and a parameter dependent quadruple of state space data 𝒜𝑘, 𝑘, 𝒞𝑘, and 𝒟𝑘, such that the following LMI constraints 𝐼𝐼𝒮<0,(11)𝛼𝑘𝑙𝐼𝐼𝒮+𝛽𝑘𝑙Φ+𝛽𝑙𝑘Φ𝑇𝑘,𝑙<0,(12)Ψ=Ψ11Ψ𝑇21Ψ21Ψ22<0(13) are satisfied for all 𝜃𝒫, ̇𝜃𝒱, with Φ=𝒜+2𝒞𝑘𝒜+2𝒟𝑘𝒞2𝒜𝑘𝒮𝒜+𝑘𝒞2,Ψ11=̇+𝒜+𝒜𝑇+2𝒞𝑘+2𝒞𝑘𝑇1+2𝒟𝑘𝒟211+2𝒟𝑘𝒟21𝑇𝛾𝐼,Ψ21=𝒜𝑘+𝒜+2𝒟𝑘𝒞2𝑇𝒮1+𝑘𝒟21𝒞1+𝒟12𝒞𝑘𝒟11+𝒟12𝒟𝑘𝒟21,Ψ22=̇𝒮+𝒮𝒜+𝒜𝑇𝒮+𝑘𝒞2+𝑘𝒞2𝑇𝒞1+𝒟12𝒟𝑘𝒞2𝑇𝒞1+𝒟12𝒟𝑘𝒞2𝛾𝐼.(14) Then, there exists a gain-scheduled output-feedback controller as (8) such that one has the following. (1)Internal stability is enforced.(2)𝛾 is a bound on the 𝐿2 gain of the closed-loop system given by the interconnection of (6) with (8).(3)The poles of each closed-loop LTI system, resulting from all constant parameter trajectories in 𝜈𝒫, are circumscribed to an LMI region prescribed by a characteristic function such as (9).

Proof. See [8, 11].

For the optimization problem to be convex, this method seeks a unique closed loop Lyapunov Matrix 𝒳 simultaneously valid for 𝐿2-gain and pole placement conditions. 𝒳 can be computed from 𝒮 and . As mentioned in [8], this approach is potentially conservative, but rarely in practice.

2.2. Main Results

Next, to proceed towards the derivation of synthesis conditions, a dependence of the so-called transformed controller matrices 𝒜𝑘, 𝑘, 𝒞𝑘, and 𝒟𝑘 on the measured parameter vector (𝜃) is proposed. Let 𝑟𝑎𝑘=(𝑟𝑎𝑘1,,𝑟𝑎𝑘𝑠) and 𝑟𝑐𝑘=(𝑟𝑐𝑘1,,𝑟𝑐𝑘𝑠)𝑠 define the numbers 𝑛𝑎𝑘 and 𝑛𝑐𝑘, and the sets Θ𝑎𝑘, ̇Θ𝑎𝑘 and Θ𝑐𝑘, ̇Θ𝑐𝑘 as in the beginning of this section. The transformed controller matrices will be given by𝒜𝑘=𝒯𝑎𝑘𝐴𝑘,𝑘=𝒯𝑎𝑘𝐵𝑘,𝒞𝑘=𝒯𝑐𝑘𝐶𝑘,𝒟𝑘=𝒯𝑐𝑘𝐷𝑘(15)

with 𝐴𝑘, 𝐵𝑘, 𝐶𝑘, and 𝐷𝑘 being constant matrices and𝒯𝑎𝑘Θ𝑎𝑘=Θ𝑎𝑘𝑇𝑎𝑘11𝑇𝒮12𝑇𝒮21𝑇𝒮22,Θ𝑎𝑘𝚯𝑎𝑘,𝒯𝑐𝑘Θ𝑐𝑘=Θ𝑐𝑘𝑇11𝑇12𝑇21𝑇22,Θ𝑐𝑘𝚯𝑐𝑘.(16)

In order to apply the FBSP on LMIs (11), (12), and (13) in a way resembling theorem 4 in [6], the following lemma is presented.

Lemma 2. Let ΘΘ, Θ𝒮Θ𝒮, ̇Θ𝒮̇Θ𝒮, ̇Θ̇Θ, Θ𝑎𝑘Θ𝑎𝑘, Θ𝑐𝑘Θ𝑐𝑘, and ΘΘ. LMIs (11), (12) and (13) can be rewritten as 𝒢𝑇(𝑀)𝒢<0,𝒢𝑇𝑝𝑝𝑀𝑝𝑝𝒢𝑝𝑝<0,𝒢𝑇𝑀𝒢<0.(17) The 𝒢, 𝒢𝑝𝑝, and 𝒢 are LFTs depending on the open loop data (6) and on the 𝒯𝒮, 𝒯, 𝒯𝑎𝑘, and 𝒯𝑐𝑘 functions. They can be expressed as 𝒢=Θ𝐺11𝐺12𝐺21𝐺22,𝒢𝑝𝑝=Θ𝑝𝑝𝐺𝑝𝑝11𝐺𝑝𝑝12𝐺𝑝𝑝21𝐺𝑝𝑝22,𝒢=Θ𝐺11𝐺12𝐺21𝐺22(18) with Θ=diag(Θ,Θ𝒮), Θ𝑝𝑝=diag(Θ𝒮,Θ,Θ𝑎𝑘,Θ𝑐𝑘,Θ) and Θ=diag(̇Θ𝒮,̇Θ,Θ𝒮,Θ,Θ𝑎𝑘,Θ𝑐𝑘,Θ). On the other hand, the 𝑀, 𝑀𝑝𝑝, and 𝑀 matrices depend on the 𝛼 and 𝛽𝑚×𝑚 matrices that specify a design LMI region, as in Definition 1, and on (1)a pair of symmetric positive definite 𝑃 and 𝑄 matrices of the 𝒮 and matrix functions,(2)a quadruple 𝐴𝑘, 𝐵𝑘, 𝐶𝑘, and 𝐷𝑘 of controller state space data,(3) a real positive performance index𝛾,where the enumerated objects are to be determined in the optimization process.

Proof. See [12] for the definition of matrices 𝑀, 𝑀𝑝𝑝, 𝑀, 𝒢, 𝒢𝑝𝑝, and 𝒢 and for the proof, which can be obtained through tedious but straightforward matrix calculations based upon the results in [6, 8].

Theorem 2 (LPV control with pole placement constraints and FBMs). The inequalities of (17) are satisfied, if and only if there exist symmetric positive definite real matrices 𝑃 and 𝑄, a performance index 𝛾, a quadruple of controller matrices 𝐴𝑘, 𝐵𝑘, 𝐶𝑘, and 𝐷𝑘, and symmetric full block multipliers Π, Π𝑝𝑝 and Π such that the following LMIs 𝑇diag{Π,𝑀}𝐺<0,𝑇diagΠ𝑝𝑝,𝑀𝑝𝑝𝐺𝑝𝑝<0,𝑇diagΠ,𝑀𝐺<0,𝐺=𝐺11𝐺12𝐼0𝐺21𝐺22,𝐺𝑝𝑝=𝐺𝑝𝑝11𝐺𝑝𝑝12𝐼0𝐺𝑝𝑝21𝐺𝑝𝑝22,𝐺=𝐺11𝐺12𝐼0𝐺21𝐺22(19) are satisfied, and for all 𝜃𝒫 and ̇𝜃𝒱, the following conditions 𝐼Θ𝑃𝑇Π𝑃𝐼Θ𝑃0,𝐼Θ𝑄𝑇Π𝑄𝐼Θ𝑄0,𝐼Θ𝑇Π𝐼Θ0(20) are met.

Proof. The application of Lemma 1 to inequalities (17) produces the desired result.

The computation of the controller's state space matrices is carried out following the algorithm prescribed in [7]. All remarks made in that paper, concerning the use of PDLFs, which aim towards obtaining controllers whose state space matrices do not depend on the parameter rate of variation, are applicable here as well (see [7, Table I]).

As observed in [6], in practice, the considerations of Remark 1 can lead to significantly reducing the number of decision variables of the problem. Degradation of the computed performance bound 𝛾 is also a possibility.

3. Application Example

3.1. Nonlinear Model

The example we consider is a sounding rocket (see Figure 1) which should follow a prescribed trajectory. A dynamic model is presented (see Figure 2), describing the position, velocity, orientation, and angular velocity errors of the actual vehicle (frame 𝐁) with respect to the prescribed trajectory (frame 𝐃). The differential gravity force is neglected in this error model for practical reasons.

A simple model of aerodynamic drag and lift forces taken from [13] is included. According to this model, these forces depend on the dynamic pressure, the angles of attack 𝛼, and sideslip 𝛽 (see [14]). The moments resulting from the aerodynamic forces are computed under the assumption that the center of pressure (CP) is located above the CM. This renders unstable aerodynamics posing a challenge on the control system (see Figure 1). 𝐹𝑥𝑎, 𝐹𝑦𝑎, and 𝐹𝑧𝑎 denote the aerodynamic forces acting on the CP in frame 𝐁.

The actuator considered for the rocket is a nozzle gimbal which allows small rotations around the 𝑧-and 𝑦-axes. A couple of gas jets are placed to generate torques around the 𝑥-axis. Hence, the actual control inputs of the plant will be the thrust, the rotation angles of the gimbal, and the torque exerted by the jets. For small rotations of the gimbal, a change of variables is carried out, and as a consequence, the control inputs of the plant model used in the controller design will be denoted by 𝐹𝑥𝑡, 𝐹𝑦𝑡, 𝐹𝑧𝑡, and 𝑚𝑥 assuming that the forces are applied at the tail (hence providing torques in the 𝑦 and 𝑧 axes). The following terms will be used to denote the actuator and aerodynamic forces and moments in the state space equations: 𝐹=(𝐹𝑥𝑡+𝐹𝑥𝑎)(𝐹𝑦𝑡+𝐹𝑦𝑎)(𝐹𝑧𝑡+𝐹𝑧𝑎)𝑇,𝑀=𝑚𝑥(𝐹𝑧𝑡+1𝐹𝑧𝑎)(𝐹𝑦𝑡+1𝐹𝑦𝑎)𝑇.(21)

𝐹𝑑 and 𝑀𝑑 denote the counterparts of 𝐹 and 𝑀 for the desired trajectory. As mass variation concerns, it is assumed that the thrust force acting on the vehicle comes from fuel exhaustion and that the inertia matrix varies uniformly with it, that is, the CM does not change. 𝐽=diag[𝐽𝑥𝐽𝑡𝐽𝑡] denotes the diagonal varying inertia matrix. The variables for the vehicle dynamics will be 𝑟, 𝑣, [𝑞0,𝑞], and 𝜔 which, respectively, give the position, velocity, orientation and angular velocity of frame 𝐁 relative to frame 𝐃. Under these assumptions, the error dynamics of the rocket arė𝑟=𝑣,̇𝑣=1𝑚𝑑𝐶𝑏𝐹𝐹𝑑,̇𝑞=12𝑞0𝜔+𝑞×𝜔,̇𝜔=𝐽1𝑑𝐶𝑏𝑀𝑀𝑑,̇𝑞0=12𝑞𝜔(22) with 𝑑𝐶𝑏 denoting the rotation matrix from frame 𝐁 to 𝐃.

3.2. Linearization: LPV Model

Linearization of (22) will be carried out, in order to obtain an LPV model of the plant, under a few extra assumptions related to nominal control and aerodynamic forces. During ascent, the vehicle should nominally follow a zero angle of attack trajectory, hence only withstanding drag. It can be shown that the nominal actuator forces acting on the 𝑦-and 𝑧-axes of the vehicle are negligible as compared to the actual forces that compensate for disturbances when the actual angle of attack deviates from zero. It can also be accepted that 𝑚𝑥 is nominally zero. Under these assumptions 𝑀𝑑=0, and 𝐹𝑑=[(𝑓𝑁𝐷)00]𝑇 where 𝑓𝑁 is the nominal thrust and 𝐷 is the drag force. Let 𝑢[𝐹𝑥𝑡𝐹𝑦𝑡𝐹𝑧𝑡𝑚𝑥]𝑇 and 𝑢𝑑[𝑓𝑁000]𝑇. Let 𝑟𝑖, 𝑣𝑖, 𝑞𝑖, and 𝜔𝑖 (𝑖=1,2,3) be the components of the corresponding vectors, and the state variable is 𝑥[𝑥𝑇1𝑥𝑇2𝑥𝑇3𝑥𝑇4]𝑇 with 𝑥1=[𝑟1𝑣1]𝑇, 𝑥2=[𝑟2𝑣2𝑞3𝜔3]𝑇, 𝑥3=[𝑟3𝑣3𝑞2𝜔2]𝑇, and 𝑥4=[𝑞1𝜔1]𝑇. The differential control is 𝛿𝑢=𝑢𝑢𝑑. With these definitions in mind we seek to linearize (22) which can be rewritten aṡ𝑥=𝑓𝑥,𝛿𝑢,𝜃=𝐴(𝜃)𝑥+𝐵(𝜃)𝛿𝑢+𝑜𝑥,𝛿𝑢,𝜃,(23) where 𝑜(𝑥,𝛿𝑢,𝜃) represents the higher-order terms of the series expansion of 𝑓 around (0,0,𝜃). 𝐴(𝜃) and 𝐵(𝜃) are the following Jacobians: 𝐴(𝜃)𝜕𝑓𝑥,𝛿𝑢,𝜃𝜕𝑥|||||(0,0,𝜃)𝐵(𝜃)𝜕𝑓(𝑥,𝛿𝑢,𝜃)𝜕𝛿𝑢||||(0,0,𝜃).(24) The LPV parameter 𝜃 is a function of mass, dynamic pressure, and the state variables of the actual vehicle. Mass variations can be estimated with a model of fuel consumption, and dynamic pressure can be estimated from inertial position and velocity measurements as well as from a model of the atmosphere. Hence, 𝜃 can be known in real time as if it were measured.

An evaluation of the Jacobian matrices of (24) through a symbolic manipulation software package shows that 𝐴(𝜃) and 𝐵(𝜃) are affine in the parameters, as follows:𝜃12𝑄0𝑚𝑆,𝜃21𝑚.(25)

It also shows that the system can be decomposed in four decoupled systems corresponding to the 𝑥𝑖 parts of the state variable 𝑥 (𝑖=1,2,3,4). As a consequence, the control problem can be split in four 𝛾-performance problems with pole placement constraints with each subsystem having a state space representation as follows (𝑖=1,2,3,4):̇𝑥𝑖=𝐴𝑖(𝜃)𝑥+𝐵𝑖(𝜃)𝛿𝑢𝑖,𝜃=𝜃1𝜃2𝑇.(26)𝜃 evolves in the parameter variation set which is a square in 2. Mass and dynamic pressure variations throughout the nominal trajectory determine the bounds for 𝜃1 and 𝜃2.

The aerodynamic forces are included in the error dynamics in a first-order approximation for zero angle of attack nominal trajectories as follows:𝐹𝑧𝑎=𝜕𝐹𝑧𝑎𝜕𝑞𝑦||||(0,0,𝜃)𝑞𝑦,𝐹𝑦𝑎=𝜕𝐹𝑦𝑎𝜕𝑞𝑧||||(0,0,𝜃)𝑞𝑧.(27)

These terms end up inside the 𝐴𝑖 matrices of the LPV plants for 𝑖=2,3 with the derivatives depending on 𝜃1. Since drag has a maximum for the nominal trajectory, its variations are neglected in the first-order approximation. Hence, aerodynamics only affect subsystems 2 and 3.

3.3. Controller Synthesis

As a consequence of decoupling, the control problem is reduced to synthesizing one controller per subsystem. This simplifies the statement since systems 𝐺1 and 𝐺4 depend only on parameter 𝜃2.

For each subsystem an augmented plant as the one in (6) must be specified. In this problem, for all subsystems, the disturbance signal is split into a part which represents disturbance forces or moments, and a part which represents measurement noise. The performance signal is divided in two parts as well. One of them involves the state variables and it represents the design's commitment with disturbance rejection at the output. The other one involves the control force and is included in order to achieve disturbance rejection with reasonable control action.

For each subsystem the augmented plant is the result of the block interconnection of Figure 3. The state space representation of each 𝐺𝑖 block is as follows:̇𝑥𝑖=𝒜𝑖(𝜃)𝑥𝑖+𝑑𝑖𝜃2𝑑1+𝑖𝜃2𝛿𝑢𝑖,̃𝑦𝑖=𝑥𝑖.(28)

For the sake of clarity as notation regards, note that the 𝑖 matrices of the systems described by (28) play the role of matrix 2 in the system described by (6). For the final augmented plants (𝑃𝑖, Figure 3) the lowpass transfer function 𝑇act(𝑠)=1/((𝑠/𝜔𝑎)+1), with 𝜔𝑎=2𝜋10rad/sec, is included in order to model actuator dynamics. As the 𝐵𝑑𝑖 matrix concerns, it is assumed for all systems that 𝐵𝑑𝑖=𝜉𝑖𝐵𝑖 with 𝜉𝑖 being a design parameter (1<𝜉𝑖<10). The 𝜉𝑖 gives the ratio between the disturbance and control forces or torques.

System 𝐺1 involves longitudinal dynamics. The matrices that make up the plant are as follows:𝐴1=0100,𝑊𝑤=5000.2,𝐵1=0𝜃2(29)

with 𝑊1=𝑊1𝑤, 𝑘𝑢1=102 and 𝜉1=10.

Systems 𝐺2 and 𝐺3 involve lateral dynamics, with their matrices being𝐴2=010000𝐷𝛼𝜃1+2𝑓𝑁𝜃20000120000,𝐴3=010000𝐷𝛼𝜃12𝑓𝑁𝜃20000120000,𝐵2=0𝜃20𝑚0𝐼𝑡0𝜃2,𝐵3=0𝜃20𝑚0𝐼𝑡0𝜃2,𝑊𝑤=500000.20000𝜋1800000.1𝜋180(30)

with 𝑊1=𝑊1𝑤, 𝑘𝑢2=𝑘𝑢3=102, and 𝜉2=𝜉3=10.

System 𝐺4 involves roll dynamics. The matrices that make up the plant are as follows:𝐴4=01200,𝑊𝑤=𝜃18000.1𝜃180,𝐵4=0𝑚0𝐼𝑥0𝜃2(31) with 𝑊1=𝑊1𝑤, 𝑘𝑢4=102, and 𝜉4=10.

3.4. Synthesis Results

The synthesis procedure was carried out on all subsystems. The pole placement region used was 𝒟={𝑧,2𝜋8rad/sec<(𝑧)}, which takes into account practical aspects such as ease of implementation and simulation. The vehicle's details can be seen in Table 1. In this example, focus is made on demonstrating the validity of the presented synthesis method. As a consequence, in order to compare the results obtained here with respect to previous work [10], SQLFs were employed on each subsystem (i.e., 𝒮 and constant). Another design decision that was taken concerns the choice of the 𝒯𝑎𝑘 and 𝒯𝑐𝑘 functions which prescribe the way controller matrices dependend on the parameter. For the example these functions were picked as 𝒯𝑎𝑘=𝐼𝑛𝜃1𝐼𝑛𝜃2𝐼𝑛,𝒯𝑐𝑘=𝐼𝑛𝑢𝜃1𝐼𝑛𝑢𝜃2𝐼𝑛𝑢.(32)

This choice was made, in order to have the same kind of affine parameter dependence of the original plant in the controller. Once the controllers for each subsystem were synthesized, they were appended to make up the complete LPV controller.

A heuristic approach to address the “fast poles” problem, while using algorithms as the one presented in [6], is to bound the trace of the and 𝒮 matrices. A bound in the range of the decision variables could be imposed alternatively. The method is occasionally successful but offers no guarantee. In this particular application, this heuristic was useless in order to prevent one single “fast pole” in the controller, per subsystem. Another heuristic was tried, consisting in the residualization of the “fast pole” in the LTI part of the controller. Nevertheless, this approach showed poor closed loop transient behavior in non-real-time simulations. As a consequence, it was dropped. On the other hand, the controller synthesized using the proposed method showed better time responses with lower overshoot. The performance 𝛾s with the standard algorithm [6] were in the order of the ones obtained with the proposed one. The overall conclusion is that the method proposed enhances the capabilities of the standard LPV technique for real-world applications.

A considerable increase in the number of decision variables can take place. For this application, with the pole placement region being a half plane, the number of decision variables of the optimization problem for each lateral controller goes from 73 with the algorithm of [6] to 1363 in this case. A more careful and potentially conservative restriction of the number of multiplier variables, in line with Remark 1, could reduce this number. This was not done in this case, given that the computation time using standard PC hardware was acceptable and considering that the number of decision variables does not influence the implementation of the controller.

With respect to previous work [10], the improvement in the performance bounds is a remarkable result (𝛾=1.89 versus 162 for subsystem 1, 𝛾=30.46 versus 96 for subsystems 2 and 3, and 𝛾=1.29 versus 169 for subsystem 4). Moreover, an improvement in the reduction of the sampling frequency from 125 to 45.45 Hz due to the feasible relocation of closed loop poles was achieved.

3.5. HIL Simulations

To stress the fact that the method is not only valid but also applicable, real-time numerical simulations were carried out. The control law was implemented in a computer based upon an Atmel TSC695E SPARC7 class microprocessor operating at 20 MHz. This 32-bit microcontroller has been available in commercial space systems for more than a decade, setting a de facto standard. The setup for simulation is fairly simple, consisting of an Atmel VAB695E Evaluation Kit, with an add-on board containing an Atmel AT7908E CANbus (ISO 11898-1) controller, connecting this computer with a standard PC featuring a Peak-PCAN pci CANbus interface board. The use of CANbus in aerospace applications is fairly standard [15]. The simulation of the vehicle's dynamics is carried out on the PC using GNU Scientific Library [16], for the real-time simulation of the vehicle’s dynamics. The PC features Xenomai Linux [17] as Real-Time Operating System. GNU Scientific Library is employed in the VAB695E software as well, in single precision floating point mode, for the implementation of the LPV control law. RTEMS real-time operating system [18] features the VAB695E board.

To evaluate the response of the system to disturbance signals the 𝐿2-gain criterion accounts for, the PC simulates sensor measurements corrupted by colored, weighted, pseudorandom noise. An input disturbance is introduced as well consisting of colored, weighted, pseudorandom lateral forces. These corrupted simulated measurements are transmitted upon request from the control computer. The VAB695E computes control inputs in real time, transmitting commands to actuators simulated in the PC. This renders simulated real time closed loop operation.

The simulations carried out show responses with the initial conditions deviated from the nominal ones as follows: 𝑥(0)=𝑟(0)𝑇𝑣(0)𝑇𝑞0(0)𝑞(0)𝑇𝜔(0)𝑇𝑇(33)

with 𝑟(0)=6378000𝟑𝟎𝟎𝟑𝟎𝟎𝑇,𝑣(0)=0.𝟏.𝟏𝑇,𝑞(0)=0.𝟎𝟑𝟏𝟔0𝑇,𝜔(0)=000𝑇,𝑞0(0)=.𝟗𝟗𝟓.(34)

All quantities in bold face should nominally be zero, except for 𝑞0 which would be nominally one. The prescribed trajectory is a vertical ascent from the north pole. Disturbance of the initial conditions is useful in order to evaluate transient behavior.

In Figure 4 the time history of the state variables can be seen. Figure 5 shows the control error forces and commands. The gimbal’s rotations (𝛿𝑦 and 𝛿𝑧) are simulated with a saturation at 5. Note nevertheless that they remain unsaturated practically throughout the whole simulation. The discrete time implementation of the controller, which is synthesized in a continuous time framework, is carried out through a zero-order hold transformation on the LTI part of the LFT controller (see [19]). The sampling frequency picked for this was 45.45 Hz (22-millisecond sampling time).

4. Conclusions

In this work, the use of FBMs was extended to LPV synthesis with regional pole placement constraints. The usefulness of the method was tested on an application example with HIL simulations. The design of an LPV controller for a 6 DOF vehicle with pole placement constraints shows an adequate response without degrading the 𝛾 performance index. The system’s LPV “poles” were satisfactorily placed acceptably increasing the computational cost of design.

Acknowledgments

This research was partially supported by the Universidad Nacional de Quilmes, Argentina, through Grant PUNQ 0530/07. The setup for simulation was assembled at the laboratories of CONAE, the Argentine Space Agency. The second author has been supported by CONICET and a PRH Grant from the Ministry of Science and Technology of Argentina. The authors wish to acknowledge the contribution of Dr. Ke Dong to this work, who gently sent the source code of the software developed for the example in [6], for us to see.